Many permeable materials made of plastics such as cellulose, acetylcellulose, teflon, polycarbonate and nylon are commercially available, including those having various pore sizes. However, the heat resistance of these materials is insufficient.
Permeable molded materials comprising a glass filter have been proposed for use as a high heat resistant filter. However, the molded materials are bulky to the extent that it is difficult to make a thin and compact filter therefrom. Permeable materials made of sintered glass are commercially available; however, these materials are so weak that thin and large products molded therefrom cannot be practically obtained. Furthermore, filters made of glass tend to easily clog.
Examples of high molecular materials which can be used for making high heat resistant molded materials include aromatic polyimides. However, because it is difficult to form a filter cloth from aromatic polyimides, a permeable filter based on such high molecular materials cannot be practically obtained. Likewise, molded filters based on aromatic polyimides also have not been practically obtained.
The preparation of molded products using polyimide powder materials is described by B. H. Lee, Modern Plastic Encyclopedia, p.62, (1988). However, this molding method is complicated, for example, because it is often necessary to first make a molded precursor material. Furthermore, when this method is applied, it is difficult to uniformly spread the polyimide powder material in a thin powder layer thickness over a wide area. Thus, it is very difficult to form thin and large permeable molded materials from aromatic polyimides.
Additionally, because the aromatic polyimides have high heat resistance and are only slightly soluble in solvents, it is difficult to prepare powders having a uniform pore size distribution therefrom.
The present inventors investigated preparation of a permeable molded material having various pore sizes. As a result, the present inventors found that such a filter comprising an aromatic polyimide could be prepared by premolding an aromatic polyimide filter into a nonwoven fabric or felt, and heating the aromatic polyimide nonwoven fabric or felt filter at a temperature higher than its glass transition temperature for an appropriate length of time as described in JP-A-6-257045 (the term "JP-A" as used herein means an "unexamined published Japanese application").
However, because polyimide has poor hydrochloric acid resistance, its filter properties quickly deteriorate when used to filter gas from the incineration of industrial and municipal wastes containing hydrogen chloride.
In order to improve its chemical resistance, a surface coating method has been proposed wherein a polyimide filter is immersed in a polytetrafluoroethylene dispersion having excellent chemical resistance, and the immersed polyimide filter is dried and heated. However, because the melting viscosity of polytetrafluoroethylene is as high as from 10.sup.11 to 10.sup.13 poise at a shear rate of 10 sec.sup.-1 and at a temperature of from 340.degree. to 380.degree. C., it is difficult to coat the whole surface of polyimide filter without leaving apertures and many pinholes. Consequently, this surface coating method does not adequately improve the chemical resistance of a polyimide filter.
Furthermore, a polyimide filter has a high electric insulating property. Consequently, when powders are filtrated using a polyimide filter in a dry atmosphere, charges develop by friction between the filter and the powders. Consequently, there is a possibility of a dust explosion upon discharging.
Therefore, in general, an antistatic agent such as glyceride is coated onto the surface of the synthetic fiber surface to inhibit charging of the fiber. However, when waste gas from incineration of industrial and municipal wastes is being filtered, or when filtering is carried out to recover original powders in a high temperature gas in the chemical industries or to collect product powders therein, antistatic agents such as glyceride tend to decompose such that their effects on filtration are of short duration.
In addition, polyimide filters have poor water vapor resistance at high temperatures. Thus, when original powders are recovered from a high temperature gas at high humidity, or when product powders are collected therefrom, the filter properties are rapidly deteriorated.